Hi,
As you know hardware firewall is a dedicated appliance which acts as as security enforcement point in your network.
The hardware resource is dedicated for that functionality.
Routers having firewall feature set also does the same job, but the hardware capability will be limited in this setup. It will not scale well.
Based on the scenario in question, the option is open for you to choose between both.
For larger deployments typically dedicated firewall appliance is the sure way to go.
-VJ